H. K. Burke and G. J. Michon describe a semiconductor imager of the charge-injection-device (CID) type in their U.S. Pat. No. 3,993,897 issued Nov. 23, 1976, entitled "SOLID STATE IMAGING APPARATUS", assigned to General Electric Company and included herewithin by reference. They also describe a semiconductor imager of the charge-injection-device (CID) type in their U.S. Pat. No. 4,241,421 issued Dec. 23, 1980, entitled "SOLID STATE IMAGING APPARATUS", assigned to General Electric Company and included herewithin by reference.
In CID image sensors that are fabricated using two levels of polysilicon, the background level signal is not absolutely flat. There is a spatial variation in the background level with no image signal present, owing to variations in threshold voltage and the interlevel capacitance from pixel-to-pixel. The interlevel capacitance varies because the dielectric is thermally grown oxide on the lower polysilicon conductors. The lower polysilicon conductors have a finite grain size, and the surface profile of the polysilicon grains is followed in the interlevel oxide, resulting in a film that is relatively uniform in thickness but non-planar in form. It is the variation from planar form that causes the variation in interlevel capacitance. This gives rise to the non-uniform, spatially-fixed background signal called fixed pattern noise. This fixed pattern noise, if not corrected, limits the sensitivity of the image sensor.
Previous apparatus corrects fixed pattern noise by reading each pixel twice, first with signal present and again after the signal has been cleared (injected). The difference between these two readings is free of fixed pattern noise within the accuracies of the readings and of the memory used to store the first reading while the second reading is being made. The memory temporarily stores a single sample of analog signal in this fixed pattern noise correction scheme. The pixel-by-pixel fixed pattern noise correction method is limited in pixel rate by the requirement that, during each scan line, each pixel be twice read and be cleared between its first and second reads.
The prior art has extended the pixel-by-pixel fixed pattern noise correction method, by reading a line of a signal is present from the CID imager and storing it for a line time interval. Then, the row is cleared of image signal and read again with only a line of fixed pattern noise present. Finally, the stored line of video and the line of just fixed pattern noise are differentially combined to recover the signal free of fixed pattern noise. The memory in such instance can be a linestore memory that stores an entire line of analog pixel values and has been implemented using a charge-coupled-device (CCD) clocked delay line. A CCD clocked delay line is difficult to make radiation-tolerant. There is a small charge transfer loss associated with each transfer of charge from one CCD clocked delay line stage to the next. The cumulative effects of these charge transfer losses begin to become significant as the number of charge transfer stages becomes appreciably large--e.g., more than fifty--leading a designer to look for a clocked analog delay line not appreciably affected by charge transfer losses. The line store method has also been implemented using two imager ports to generate dynamic updates to the fixed pattern noise.
A full frame memory has been used to store an entire frame of background fixed pattern noise samples for substraction from respective ones of the sensor output samples. The frame store method relies upon the fixed pattern noise being stable between frame store updates. A frame store for analog pixel samples can be implemented using CCD memory, with radiation-tolerance problems similar to those for a CCD clocked delay line. The frame store method requires a large memory and a method to update the fixed pattern noise frame as conditions, such as radiation-induced changes in image sensor pixel threshold voltage, change.
To avoid the problems of radiation-intolerance in CCD memory, techniques for digitizing the analog pixel samples and storing the digitized signals in digital memory have been used in the prior art. The digital memories tend to require considerably larger semiconductor die areas than analog memories, and errors are introduced in the digitization procedure.
One line of video memory is required for fixed pattern noise correction in CID star sensors operated in the row readout mode. It is desirable to include this line of video memory as analog memory on the sensor integrated circuit, in order to reduce overall star sensor apparatus size. Including this analog memory on the sensor integrated circuit in a radiation-tolerant star sensor also reduces the number of parts that need to be qualified for radiation tolerance. Linear capacitors and multiplexing switches are used to implement this analog memory in the invention.